With Inkscape, it's easy to do a map or at least block out areas for a map.
I took the photo and imported it into Inkscape. My next step was to decide what size I wanted the image to be. I picked about 8.5" by 11". This gives me all of the white space around the castle to build the exterior areas which might be important to the user.
So, how does Inkscape help build a map from a photo of a drawing?
My first step was to make a series of rectangles the same size as the tower bases in the image. As I did each rectangle, I duplicated it and turned the duplicate 90 degrees. I did not adjust for the depth-wise adjustment of the towers. I could have but didn't want to make it too complex.
The corner of one rectangle met the corner of its duplicate, leaving an open square. Once I had done that for every box, I tried to do the same for the central palace-like area.
Once I was done, I put a red box or rectangle in that open space between the two grey rectangles. This allows me to map out a proportional arrangement of the structures with no measuring of anything. I deleted the grey rectangles and roughed out the walls between the towers. This is far from a perfect match, but it is very close.
On the right-hand side of the map, you can see that I moved one tower very significantly. I just thought it looked better. Also, the drawing shows a series of buildings that divided the structure in half. I removed them so there would be an open space inside.
Of course, some of my towers are circular. I simply replaced a few squares with circles. The trick here is to make the circles slightly larger than the squares they replaced.